Problem: Before the previous stop there were 15 people riding on a train. 6 people ended up getting off at the stop. How many people are riding the train now?
Answer: The number riding the train now is the difference between the number who were riding and the number who got off at the previous stop. The difference is $15 - 6$ people on the train. $15 - 6 = 9$ people are on the train.